Some punctuation marks prompt the reader to give a word or sentence more than usual emphasis. For example, a command with a period does not evoke the same emphatic response as the same command with an exclamation mark.

Watch out!

A dash or colon has more emphatic force than a comma.

The employees were surprised by the decision, which was not to change company policy.

The employees were surprised by the decision—no change in company policy.

The employees were surprised by the decision: no change in company policy.